The combustion of a fuel source, such as coal, oil, gas, wood, municipal waste, industrial waste, hospital waste, hazardous waste and agricultural waste, in furnaces or boilers generates hot flue gases that contain combustion products such as carbon monoxide, carbon dioxide, nitrogen oxides, sulfur compounds, and other contaminants. Included among these other contaminants are particulates. The particulates may include fly ash, dust, smoke, and other fine particulate matter that can comprise phosphorous, heavy metals, alkali metals and alkaline earth metals. The nitrogen oxides (NOx) contained in the hot flue gas streams include nitric oxide (NO) and nitrogen dioxide (NO2). The sulfur compounds include the sulfur oxides (SOx) such as sulfur dioxide (SO2) and sulfur trioxide (SO3). The sulfur compounds result from the presence of sulfur in the combustion fuel.
A common method for removing NOx from the flue gas streams of combustion processes is the selective catalytic reduction (SCR) process. This process involves the catalytic reduction of NOx to nitrogen (N2) and water (H2O) by reaction of NOx with ammonia (NH3) within a catalyst bed. The primary reactions of the SCR process are presented as follows:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O6NO2+8NH3→7N2+12H2O
The catalyst bed usually includes a catalytically active material, such as a nitrogen oxide decomposition catalyst, also referred to herein as deNOx catalyst, that can comprise a metal oxide and a catalytically active metal component such as titanium, tungsten, molybdenum, vanadium or other suitable compounds known to catalyze the conversion of nitrogen oxides to molecular nitrogen and water. Examples of catalytically active materials are vanadium pentoxide (V2O5) and tungsten trioxide (WO3).
One problem with the use of the deNOx catalyst of the SCR process in treating combustion flue gas streams is that over time they become contaminated and deactivated by the deposition of particulates and reaction products of ammonia with the sulfur compounds of the hot flue gas stream. These products include, for example, ammonium sulfate and ammonium bisulfate. Other ammonium salts, such as ammonium chloride and ammonium nitrate, formed by the reaction of injected ammonia with components of the flue gas stream, also may deposit on the deNOx catalyst. When the deNOx catalyst becomes deactivated due to deposition of ammonium salts, there is a need to regenerate the catalyst to restore at least a portion of its lost activity.
U.S. Pat. No. 8,883,106 describes one method of regeneration of deNOx catalyst. This patent presents a selective catalytic reduction reactor system for removing nitrogen oxides and sulfur oxides from hot process gas. The reactor system has structural features that provide for an on-line process for regenerating its catalytic elements. This system includes multiple catalyst bed segments arranged in parallel with the flow of the hot process gas that is treated by use of the system. The patent further discloses a method of regenerating the catalyst bed segments. The regeneration method includes isolating one of the catalyst bed segments from the flow of hot process gas and passing a regenerating gas through the isolated catalyst bed segment while the other catalyst bed segments are in simultaneous use to remove nitrogen oxide and sulfur oxide from the hot process gas.
EP 2 687 283 describes another method of regeneration of deNOx catalyst. This publication shows a gas treatment system or facility used for nitrogen oxide removal from a gas stream by catalytic reduction of the nitrogen oxides contained in the gas stream. The gas treatment system includes a reactor system having multiple separate reactors or compartments with catalyst structured to allow for the regeneration of the catalyst of an individual reactor or compartment while using the other reactors or compartments with catalyst in the treatment of the gas stream. The system further includes a dechlorination/desulfurization unit that is located upstream of the reactor system and provides for the treatment of the gas stream. The system also includes a gas treatment circuit and a regeneration circuit. The gas treatment circuit provides for the denitrification of the gas stream by supplying the gas stream to and through the catalyst modules of the reactor system while the regeneration circuit provides for the regeneration of a portion of the catalyst of the reactor system by circulating a regeneration gas through its other catalyst modules. The regeneration off-gas is combined with the gas stream fed to the dichlorination/desulfurization treatment.
Some of the problems with these prior art flue gas catalytic denitrification systems that provide for on-line methods of catalyst regeneration arise from the systems having equipment that is structured with separate reactors or compartments. These separate reactors or compartments are isolated from each other to allow for regeneration of a single reactor or compartment simultaneously with the use of the remaining reactors or compartments in treating the flue gas stream. These regeneration methods require complicated structural features that include separate reactors or compartments as well as valving and switching systems that are expensive and difficult to use and to control.
It is an ongoing desire to provide improved catalytic gas treating systems that are easier to use and require less cost than many of the prior art systems.